Meet Spica, the Ear of Grain

May 6, 2019 - Comment

The Vitals (for Spica A) Official title (IAU-approved) Spica Different Designations Alpha Virginis, Spica A Nicknames — Obvious Magnitude zero.98 Distance from Earth ≈ 250 light-years Kind Blue big, B1 III-IV Colour Bluish Mass ≈ 10 M☉ Radius ≈ 7 R☉ Constellation Virgo Proper Ascension 13h 25m 11.6s Declination -11° 09′ 41” A number of

The Vitals (for Spica A)

Official title (IAU-approved)
Different Designations
Alpha Virginis, Spica A

Obvious Magnitude
Distance from Earth
≈ 250 light-years
Blue big, B1 III-IV
≈ 10 M☉
≈ 7 R☉
Proper Ascension
13h 25m 11.6s
-11° 09′ 41”
A number of system?
Variable star?
Exoplanets standing
None recognized
Possible destiny
Unsure, however possible supernova

Bodily Traits

Spica (left) paired carefully with vivid Venus (proper) in November 2018.
Marion Haligowski / S&T On-line Photograph Gallery

Spica is the brightest star within the constellation Virgo, and one of many high 20 general brightest stars seen to us, so it’s fairly well-known. But Spica isn’t a singleton: it’s really two stars.

Binary stars, after all, aren’t any strangers to beginner astronomers. They’re fairly frequent, and plenty of are simply seen by means of a telescope or binoculars (and even with out optical support, similar to Mizar and Alcor). So it shouldn’t come as a shock to listen to that Spica is definitely a pair of stars — besides which you could’t see the second star, even with an enormous telescope. The system is an instance of a spectroscopic binary. As the celebrities orbit one another, the sunshine they emit shifts barely blueward or redward relying on whether or not they’re transferring towards or away from us, respectively. These so-called Doppler shifts are what astronomers picked up in 1890, suggesting that Spica A had a companion.

Visually, although, the pair merges right into a single level. Why is that this? Distance is partially accountable, as Spica is greater than 260 light-years from us. However the different cause is that the 2 stars are extraordinarily shut collectively, separated by a decent 11 million miles, or zero.1 astronomical unit. That’s solely a 3rd of the typical distance between Mercury and the Solar. The celebrities revolve quickly round one another with an orbital interval of solely 4 days.

Spica’s mild curve as measured by MOST. The inexperienced line is the anticipated curve with no eclipses. The purple line reveals a mannequin of the system with eclipses. The eclipses seem as periodic downward spikes on the section the place we see the egg-shaped stars end-on, with the dimmer one in entrance.
MOST Group

The celebrities’ proximity to one another causes some intense tidal forces. Simply because the Moon’s gravity barely reshapes Earth, inflicting it (largely its oceans) to bulge on the equator, Spica A and B exert tidal forces on one another however in a much more spectacular vogue. Regardless that the 2 stars are separated by a distance higher than that between Earth and the Moon, they’re additionally fairly huge in order that they exhibit a robust gravitational pull on one another. These forces are so intense that the 2 stars distort one another’s form.

Due to the celebrities’ oval shapes, the system brightens and fades as they orbit one another. These variations aren’t fairly common, although, as a result of Spica A additionally pulsates, driving further, speedy adjustments in brightness. From Earth’s viewpoint, the pair cross over one another simply barely in a grazing eclipse, which provides one other supply of variability.

Spica A has a mass of about 10 occasions the Solar’s, whereas its smaller companion Spica B about six occasions extra huge than the Solar. The vast majority of the system’s luminosity, nevertheless, comes from the principle star.

Origin / Mythology


Virgo, as illustrated in Johannes Hevelius’s Prodromus Astronomia. Spica is the star within the sheaf of grain in her left hand.

Virgo, Spica’s constellation, has many alternative myths connected to it from numerous cultures — Greek, Babylonian, Egyptian — however they typically focus on two frequent factors: 1) Virgo is a lady, and a pair of) she has one thing to do with spring, crops, or In some illustrated star charts, Virgo is drawn with a bundle of wheat or another grain in her hand. This grain marks the situation of Spica, which is certainly Latin for “ear of grain.” This affiliation with harvests could come up as a result of the Solar strikes into Virgo within the fall. The affiliation with spring is attention-grabbing as nicely, as Virgo first turns into seen within the evenings proper across the spring equinox.

Within the 100s B.C., Spica helped the Greek astronomer Hipparchus uncover the precession of Earth’s poles when Hipparchus visited an historic Egyptian temple in Thebes. The temple had been constructed about 2,000 years prior and was regarded as oriented based on Spica’s heliacal rising — the time of 12 months when the star turns into briefly seen above the jap horizon simply earlier than dawn. Noting that Spica had “drifted” throughout the passing millennia led Hipparchus to theorize the gradual drift of precession.

See Spica

Seen from North America, Spica is in a superb place for commentary all through the spring — when it begins to rise at about sundown — and thru the primary half of summer time, when it’s seen pretty excessive within the southern sky. It’s near ecliptic, so anticipate to view occasional conjunctions with the Moon and/or planets, in addition to the uncommon occultation.

Brief return to the present

From the Huge Dipper’s deal with, “arc to Arcturus,” then “velocity on to Spica.”
Supply: Stellarium

Discovering Spica is straightforward, as soon as the mnemonic: Following the Huge Dipper’s deal with, arc to Arcturus, then velocity on to Spica. The space between the Huge Dipper and Arcturus is roughly the identical as between Arcturus and Spica. The route is fairly a protracted one to hint, however a mild curve will get you there.

Why not exit and consider Spica this spring? Whilst you’re doing so, take a second, as many have completed earlier than you, and recognize the best way this “ear of grain” heralds the season of development and planting.

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